Separate from the question of efficiency/game speed, is the impact on gameplay of veterancy bonuses
A - get rid of veterancy bonuses for everything except ACUs. ACUs getting veterancy is an important part of how FAF is balanced, taking that way could cause problems
B - for ACUs, veterancy bonuses shouldn't restore HP equal to the amount of max HP gained. It should only restore a percentage of the health increase (equal to the ACU's % of health). E.g. if your ACU goes from 10k max HP to 11k max hp, you would gain 1k max HP of course, but you would only gain 10% of your current HP. If the ACU is at 2000 health, it would bump up to 2200. Veterancy would still be very useful for ACUs, but it wouldn't be nearly as helpful to survive a kill attempt. (in that situation, you would only get 200 hp instead of 1000)
That would make the game better.
Then after that change is implemented we could decide how much slowdown we want to accept in exchange for having veterancy tracked. Veterancy would still be useful, for example if you are watching a replay you can get a sense of how effective your units are based on how much vet they have. But we could decide if certain things need to be tweaked, like changing the way ASFs get vet to avoid that particular slowdown. We could have ASFs only get vet from delivering the killing blow on another ASF (and maybe all ASF in the vicinity get credit when an enemy T4 air unit dies for any reason).
Also it would be nice if SMDs and TMDs gained "vet" by shooting things down. E.g. SMDs get a vet level every time they kill a nuke and TMDs get a vet level every time they kill 100 missiles. Again the point is really just sentimental not to change gameplay.
And separately, it would be nice if there was a little window showing the "total mass killed" (the veterancy figure) summed up for all selected units. So if you selected 100 tanks, and you have unit selection cost UI, you can see that they cost 20k mass, and you could see that the surviving tanks had 5k mass killed, that would be pretty cool.